The present invention relates to an optical imaging system for electrophotography.
In a typical electrophotographic apparatus, a converging lens projects a light image of an original document along an optical path onto a photoconductive drum or belt. Prior to this imaging operation the drum is electrostatically charged and the light image causes localized photoconduction which results in the formation of an electrostatic image on the drum. A toner substance is applied to the drum which produces a toner image which is transferred and fixed to a copy sheet to provide a permanent copy of the original document.
Whereas known apparatus of this type are capable of producing acceptable copies of black and white documents, the quality of reproduction of colored documents is dependent on the colors of the documents. More specifically, the photoconductive coating on the drum has a spectral sensitivity characteristic such that the sensitivity of the coating for various wavelongths of light differs greatly. Such coatings are especially sensitive to blue light, and the sensitivity to orange and red light may be close to zero. Whereas a document with a white background and red printing would be reproduced with good contrast, a copy of a blueprint would be almost completely white since the spectral sensitivity of the coating for blue light is almost as great as for white light. Attempts to overcome this problem, especially to increase the spectral sensitivity for red light comprise the addition of a suitable sensitizing substance to the coating which makes it more sensitive to red light. Suitable additives have not yet been produced, however, which will both increase the sensitivity to red light and decrease the sensitivity to blue light to the required extent.
The expendient which is currently employed in order to obtain satisfactory reproductions of blueprints and other documents comprising blue areas is to place a suitable filter in the optical path of the converging lens. However, while sufficient contrast is obtained using this method, the amount of light lost through absorption by the filter is a serious problem which has heretofore remained unsolved.
Where two filters are used together for sensitivity compensation of a photoconductive coating which has been sensitized for red light, the amount of light loss may be greater than 30%.